Method for carrying out surface plasmon resonance measurement and sensor for use in the method

ABSTRACT

In a method for carrying out a surface plasmon resonance (SPR) measurement a beam of electromagnetic radiation is directed through a part transparent to it onto a surface of a material layer having been brought on its opposite side in contact with a test material. A change in the intensity of the reflected radiation, caused by the resonance phenomenon, is utilized for analyzing the test material. The material layer is of catalytic material, for example palladium, having a negative real part of the dielectric constant at the used wavelength of the electromagnetic radiation and being capable of catalyzing chemical reactions in which the test material takes part. The measurement is carried out at such a wavelength and angle of incidence of the radiation onto the surface that a change resulting from the material concentration accumulated on the opposite side of the material owing to the catalytic properties is detectable in the intensity of the reflected radiation.

The invention relates to a method utilizing surface plasmon resonancephenomenon for analyzing various substances, as well as to a sensor forcarrying out the method.

BACKGROUND OF THE INVENTION

The surface plasmon is a particular kind of electromagnetic wave whichpropagates along the surface of a metal (H. Raether, "Surface plasmonson smooth and rough surfaces and on gratings", Springer-Verlag, Berlin,1988). Optical excitation of the surface plasmon can be achieved if ap-polarized, collimated light beam undergoes total reflection on thesurface of glass substrate coated with a thin metal film (so-calledKretschmann configuration). The momentum of photons should match thesurface plasmons on the opposite surface of the metal film in order tomake this possible. This occurs for a given wavelength at a givencritical angle of incidence of light. The phenomenon is observed as asharp minimum in the intensity of the reflected light when the angle ofincidence is varied. The angle or wavelength at which this dip occursdepends decisively on the properties of the surface layer on top of themetal film, and therefore the phenomenon can be used to monitor changeson this surface layer caused e.g. by a specific chemical or biologicalreaction or by the change of concentration of some substance in theimmediate vicinity of this surface.

In principle, any material having a negative dielectric function can beused for the excitation of plasmons Most metals fulfill this requirementin the range of visible wavelengths. An ideal material (SPR-material)should have a negative coefficient of the real part of the dielectricconstant and as large an absolute value of this coefficient as possibleand at the same time as small an absolute value of the coefficient ofthe imaginary part as possible. In the previous methods utilizing theSPR-phenomenon the choice of metals and their thicknesses has been madeusually to satisfy the criterion for the sharpest peak with as total anextinction of the intensity of reflected light as possible. For thisreason the metals most used are silver and gold, silver giving thesharpest peaks. Because of the steep curve the sensitivity of the methodis good when a constant angle of incidence is used. When the aim is toimprove the dynamics of the method, several angles of incidence can beused additionally for carrying out a simultaneous intensity measurement.The method of the above kind is disclosed for example in GB-PatentApplication 2,197,065. It is also possible to observe the location ofthe peak, that is, the measurement gives more data which are independentof each other.

The SPR-material is in addition coated with a specific surface layerhaving certain affinity towards the test material to attain desiredsensitivity. The changes occurring in this layer form the basis for themeasurement.

The purpose of the invention is to present an improvement for theabove-mentioned methods and to present a method and sensor making itpossible to widen the field of use of the SPR-method. When acatalytically active material is used as the material, the catalyticproperties and the SPR-properties of the material can be combined in away that creates new possibilities of analysis.

The method and sensor can also make use of materials having a relativelylarge optical absorption. This kind of material, even if it creates abroad peak, can be used in SPR-sensors on the prerequisite that theanalyses are carried out at an angle of incidence and wavelength thatare properly chosen.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following more closely withreference to the accompanying drawings, wherein;

FIG. 1 shows a typical resonance curve obtained using the sensormaterial of the invention;

FIG. 2 illustrates the principle of the method in accordance with theinvention;

FIG. 3 shows a test carried out with a sensor in accordance with theinvention;

FIG. 4 shows a test carried out with a traditional sensor; and

FIG. 5 shows a sensor in accordance with the invention and the auxiliaryequipment associated therewith.

DETAILED DESCRIPTION OF THE INVENTION

The success in a SPR-measurement depends mainly on two factors.

The material of the material layer must be SPR-compatible as such, thatis, it must possess some predetermined dielectric properties discussedhereinabove. For ensuring good sensitivity the curve illustrating theintensity as a function of the angle of incidence must have at least oneportion that is steep enough so that at this point a small shift of thecurve which takes place owing to the changes on the opposite surface ofthe material layer would cause a change of intensity that is as large aspossible.

On the other surface of the material layer the test material must effecta change to cause a sufficiently large change of intensity. This kind ofchange can be caused by a change in the dielectric properties of thesurface resulting from the change of concentration of a substance in thevicinity thereof.

In accordance with the invention, the material layer for the measurementis so chosen that it on one hand is SPR-compatible material as such, andon the other hand its opposite surface facing away from the radiationconstitutes an area in which changes in properties influenced by thetest material take place owing to the fact that the material layer is ofcatalytic material being capable to act as a catalyst for some chemicalreaction in which the test material takes part. Even if the reaction didnot happen on said surface, the catalytic properties of the materialtowards the test substance will cause the accumulation of the testmaterial on the surface of the material layer in a concentration beingsufficiently high to make it possible to detect the change in theSPR-conditions using measurement techniques.

It is clear that on one hand there exist several catalytically activematerials, such as metals, that at the same time are SPR-compatible. Onthe other hand there exist several substances that take part in areaction that at least one SPR-compatible material is capable ofcatalyzing in the heterogenous catalysis, in which the catalyst and thereactive material are in different phases. The heterogenous catalyst tobe used in the method can be chosen according to the test material andthe SPR-compatibility of the catalyst. Examples of some materialscommonly used in the heterogenous catalysis and having a negative realpart of the dielectric constant are titanium, cobolt, nickel, platinum,aluminum palladium. The properties that have been too poor fortraditional SPR-methods (e.g. the lack of a clear peak at the angle ofincidence causing a sharp intensity minimum) are no more a bar when saidmaterials will be applied in accordance with the present invention. Itmust also be noted that even if pure metals are listed above, use can bemade also of catalytically active metal alloys and even semiconductors,provided that they have a negative real part of the dielectric constant.The table below lists the dielectric constants of some metals at givenwavelengths.

                  TABLE                                                           ______________________________________                                        Optical constants for some metals                                                                          dielectric                                                                             SPR-                                    Metal   wavelength (nm)      constant ratio                                   ______________________________________                                        Ag      632.8        -18.22+i    0.48 37.96                                   Cu      632.8        -14.67+i    0.72 20.38                                   Au      632.8        -10.92+i    1.49 7.33                                    Al      650.0        -42.00+i    16.40                                                                              2.56                                    Pd      620.0        -14.40+i    14.60                                                                              0.99                                    Pt      640.0        -11.10+i    15.70                                                                              0.71                                    Ni      620.0        -9.60+i     14.09                                                                              0.68                                    Co      617.0        -12.10+i    18.00                                                                              0.67                                    Pb      650.0        -8.67+i     13.40                                                                              0.65                                    Ti      617.0        -6.71+i     19.86                                                                              0.34                                    Fe      632.8        -1.02+i     17.81                                                                              0.06                                    Cr      617.0        -0.84+1     20.92                                                                              0.04                                    V       617.0        3.41+i      21.38                                                                              0.16                                    W       636.0        4.30+i      21.32                                                                              0.12                                    ______________________________________                                         SPR-ratio = the real part of the dielectric constant provided with an         opposite sign divided by the coefficient of the imaginary part           

The wavelength and the angle of incidence of the electromagneticradiation used in the measurement can be chosen according to theSPR-material and the test material. Light is as a rule mentioned inconnection with SPR-methods, but it should be noted that some suitablewavelengths can be situated outside the visible light within theIR-range.

As shown by FIG. 1, for the metals with a relatively large opticalabsorption the peak of the resonance curve (intensity as function ofangle of incidence) is broad and the point of minimum is thereforeindefinite. According to the invention the metals having such a broadSPR-response are useful in SPR-measurements if advantage is taken of thecatalytic properties of the metals. Consequently, the measurement canalways be carried out with a good sensitivity at such an angle ofincidence for which the curves illustrating the intensity as a functionof the angle of incidence are as distinctly separated from each other aspossible in the measurement range for the test material. In this case,for example, the down-way slope of the resonance curve can be used, saidslope being effected most by the changes in the resonance conditions.The determination of the shape of the whole curve and the location ofthe minimum point is not necessary if the measurement is performedwithin a defined concentration range.

Alternatively, the speed of change of the intensity value can bedetermined instead of the concentration determinations by means of theabsolute intensity values, that is, the time derivative of the intensitystarting from the moment when the surface came into contact with thetest material is determined. This will indicate the concentration of thematerial even if at the final stage of the measurement the flat portionof the curve had shifted to the range of the used angle of incidence.

FIG. 2 shows an experiment conducted with one of the materials of theinvention, palladium. A 13.4 nm thick palladium layer was coated withmonomolecular Cd-behenate layers in such a fashion that the number ofthe layers was increased from 2 to 8. A SPR-measurement was conductedafter the addition of each pair of layers and the curves representingthe intensity as a function of the angle of incidence were registered.The thickness of one Cd-behenate layer was 3 nm. In FIG. 2, a curveobtained with a bare palladium surface is denoted by letter A, the curveof two layers by letter B, the curve of four layers by letter C, and thecurve of eight layers by letter D. It can be clearly seen in FIG. 2 thatthe changes taking place on the surface shift considerably the locationof the down-way slope portion of the curve. Consequently, as themeasurement is carried out at a suitable angle of incidence that islocated within the downway slope portion in each of the curves obtainedwithin the measurement range of the test material, the slope portionsbeing well enough separated from each other, a maximum change of theintensity values and a good sensitivity is obtained as a result. FIG. 2reveals that materials not used as SPR-materials because of certainprejudices can well be used also for this purpose if advantage issimultaneously taken of their catalytic properties.

Because palladium can be used as SPR-material it brings some newpossibilities for SPR-analysis. The catalytic properties of palladiumtowards substances containing hydrogen make it very interestingconsidering the analysis of these substances. Palladium dissociateshydrogen on its surface and it has good affinity to some substancescontaining hydrogen. This feature has been utilized previously insemiconductor sensors measuring the concentrations ofhydrogen-containing molecules. Substances that can be measured by meansof palladium in these types of sensors are hydrogen sulphide, alcohols,ethylene etc. (J. Lundstrom, M. Armgarth, A. Spetz, F. Windquist, "Gassensors based on catalytic metal-gate field-effect devices", Sensors andActuators, 10, 399-421 (1986)).

FIG. 3 shows a SPR-test conducted with palladium. In the test hydrogengas, air and helium were led to the surface of palladium in succession.The curve of FIG. 3 indicates the intensity value obtained by mean ofthe method of the invention as a function of time. As is apparent fromthe Figure, the sensitivity of palladium to hydrogen is clear and inaddition the phenomenon is reversible. A special phenomenon is the riseof the intensity value caused by hydrogen. Because hydrogen has asmaller refractive index than air, the intensity values should decreaseowing to the gaseous hydrogen led to the vicinity of the palladiumsurface. However, the intensity values raise considerably, whichindicates the fact that hydrogen changes the properties of the surfacelayer of palladium itself owing to the catalytic properties ofpalladium. Moreover, only a minor pit caused by helium can be observedin the curve, which illustrates well the selectivity of palladiumtowards hydrogen.

FIG. 4 shows in the scale of FIG. 3 the SPR-test conducted with gold,the conventional material. A considerable difference compared withpalladium is that hydrogen causes the decrease of the intensity values,as expected. The results with other gases, carbon dioxide and helium,are also in conformity with the expectations.

FIG. 5 shows a sensor that is particularly well suitable in the methodaccording to the invention. The sensor comprises a monochromatic lightsource 1 and collimating optics 2 provided for making the light beamemanating from the light source 1 parallel and for directing it througha polarizer 3 to meet the surface 5a of the metal layer in apredetermined angle of incidence φ. The metal surface is constituted ofthe inner surface of a metal film fixed on transparent dielectricmaterial 4, a glass prism. The total reflection taking place on thissurface leads the light beam to a detector 6. The light beam 1, theoptics 2 and the prism 4 are so disposed that the collimated light beamstrikes the interface 5 of the prism and the metal layer at an optimumangle of incidence φ, which is within the measurement range of the testmaterial in such a fashion that the steep portions of all the curvescorresponding to the concentration values essential for a successfulmeasurement are situated at the angle of incidence. In this way it ispossible to obtain an intensity value or a time derivative of theintensity value that is proportional to the concentration.

On the other side 5b of the metal layer 5 is situated the material to beanalyzed. In the case of palladium as the metal layer the material issome substance containing hydrogen. For example, a through flow cell canbe used at this area and this area can be also isolated from thesurroundings by means of a selective isolating layer 8, such as amembrane that passes only the substance to be analyzed through if thesurroundings which are examined contain other substances towards whichthe material of the material layer shows catalytic properties. Aradiation heater preventing the condensing of water molecules on thesurface of the metal layer, known as such, is denoted by referencenumeral 7 in the Figure.

An exact value for the angle of incidence φ can be calculated on thebasis of several factors such as the thickness of the metal layer, thewavelength of light and the dielectric constants of the dielectricmaterial 4 and metal 5 at this wavelength. For example, as palladiumserves at the material of the metal layer 5 and He-Ne-laser serves asthe light source at the wavelength 632,8 nm and with a prism of BK-7optical glass, the thickness of the palladium layer should be 13 nm andthe angle of incidence 42,7°.

The accuracy and selectivity can be improved by using also a so-calleddifferential measurement having two light beams emanating from the samelight source, one of them directed to an active part of the SPR-surfaceand the other of them to an inactive part of the same surface. Theintensity difference of the reflected light beams is sued as themeasurement data rather than the absolute intensity.

The so-called fixed-angle-method has been described hereinabove (onlyone angle of incidence), which can be realized with a sensitive andinexpensive sensor. However, the invention can also be applied in amethod that utilized several angles of incidence simultaneously for abetter measurement dynamics. The invention can also be applied forexample in the sensor type disclosed in a previous patent application901186 by the Applicant.

All the measurement data obtained in the above methods can be processedby means of a microprocessor 9 connected to the detector and being alsocapable of modulating the light sources and synchronizing the detector.

In all of the above cases sensor can be easily miniaturized by usingavailable electro-optical components (LED:s, detectors, heaters) andfiber optical connections.

The method according to the invention makes it possible to use newsensor material and to analyze new substances by means of measurementsbased on the SPR-phenomenon. The invention can be applied in a very widerange, such as medical diagnosis, wood industry, process control,monitoring the condition of the environment, detecting leaks etc.

I claim:
 1. A method for carrying out a surface plasmon resonance (SPR)measurement comprising:directing a beam of electromagnetic radiationthrough a part transparent to said radiation onto a surface of amaterial layer for reflection of said radiation therefrom, said materiallayer on its opposite surface being in contact with a test material; andmeasuring the change of intensity of the reflected radiation caused bythe resonance phenomenon for analyzing the test material, wherein thematerial layers is a layer of a catalytic material having a negativereal part of the dielectric constant at the used wavelength of theelectromagnetic radiation of SPR-compatibility, and is capable ofcatalyzing a chemical reaction in which the test material takes part,the measurement being carried out at such a wavelength and angle ofincidence of the radiation onto the surface that a change resulting fromthe contact of the test material with the opposite surface of saidmaterial of the material layer owing to the catalytic properties of thematerial is detectable in the intensity of the reflected radiation.
 2. Amethod as claimed in claim 1, wherein said material layer consists of ametal catalyst.
 3. A method as claimed in claim 2, wherein the materiallayer is selected from the group consisting of titanium, cobalt, nickel,platinum, aluminum, palladium and mixtures thereof.
 4. A method asclaimed in claim 3, wherein the material is palladium.
 5. A method asclaimed in claim 4, wherein the test material contains hydrogen.
 6. Amethod as claimed in claim 5, wherein the test material is selected fromthe group consisting of hydrogen gas, ammonia, hydrogen sulphide, analcohol and a hydrocarbon.
 7. A sensor for carrying out a surfaceplasmon resonance (SPR) measurement, comprising:a source ofelectromagnetic radiation; a material layer having a surface exposed toelectromagnetic radiation from said source and an opposite surface;means for bringing a test material in contact with the material on saidopposite surface; means for directing the source of radiation relativeto said surface of the material layer in such a fashion that theradiation meets said surface at an angle of incidence enabling a surfaceplasmon resonance phenomenon; a detector for measuring the intensity ofthe light beam which has been reflected from the surface of the materiallayer and has undergone said resonance; and a device for treating theintensity values, said material layer being catalytic material having anegative real part of the dielectric constant a the used wavelength ofelectromagnetic radiation of SPR-compatibility and being capable ofcatalyzing a chemical reaction in which the test material takes part. 8.A sensor as claimed in claim 7, wherein said material layer consistsessentially of metal catalyst.
 9. A sensor as claimed in claim 8,wherein the material of the material layer is selected from the groupconsisting of titanium, cobalt, nickel, platinum, aluminum, palladiumand mixtures thereof.
 10. A sensor as claimed in claim 9, wherein thematerial is palladium.